Efforts are being made in the automotive industry to develop active wheel suspension systems based on the concept of skyhook control. An active suspension system controls the distance between the vehicle body and road surface at a point corresponding to each of the wheels and the damping force of each damper depending on the road condition and the static and dynamic state of the vehicle on a real time basis. Therefore, an active suspension system can provide such advantages as minimization in changes in the attitude of the vehicle, an improved ride quality and prevention of bouncing of wheels, and these also contribute to overall improvements in motion stability and drive/brake performance of the vehicle. An active suspension system requires information on the yaw rate and lateral acceleration of the vehicle for its control action, and obtains such information from corresponding sensors (such as a lateral G sensor and a yaw rate sensor). Such signals are often contaminated with high frequency noises of various origins. It has been therefore customary to use a low pass filter having a certain cutoff frequency to remove high frequency noises from the signal of a lateral G sensor or a yaw rate sensor to ensure a high reliability in controlling the motion of the vehicle. Such examples can be found in Japanese patent laid open publications 07-25257 and 06-278638.
However, such arrangements are known to have a shortcoming because the low pass filter indiscriminately attenuates all the frequency components that are higher than the cutoff frequency in each case. For instance, in a motion control of a road vehicle, when a steering wheel is turned rapidly, the detection signal of the yaw rate sensor or lateral G sensor may change very rapidly. However, because the low pass filter eliminates high frequency components of the sensor signal, such rapid changes in the sensor signal is lost so that the control system is unable to produce a response that would suit such a situation. On the other hand, if a higher cutoff frequency is selected so as to avoid the loss of sudden changes in the detection signal, high frequency noises may not be adequately removed, and a reliability in the control action cannon be achieved.